This invention relates generally to improvements in silicon crystal growth processes and, particularly, to a vision system and method for measuring the size and size distribution of polycrystalline chunks for charging a Czochralski silicon crystal growth process.
Single crystal, or monocrystalline, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polysilicon, or polycrystalline silicon, in a quartz crucible located in a specifically designed furnace. Typically, the charge is made up of irregularly-shaped chunk polycrystalline silicon prepared by, for example, the Siemens process. The preparation and characteristics of chunk polysilicon are further detailed in F. Shimura, Semiconductor Silicon Crystal Technology, pages 116-121, Academic Press (San Diego Calif., 1989) and the references cited therein. After the polysilicon charge in the crucible is melted, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt.
A substantial concern in the production of single crystal ingots by the Czochralski process is the need to prevent the formation of dislocations, voids, or other defects in the single crystal lattice structure. In general terms, dislocations are undesirable faults in crystal geometry resulting from thermal shock, vibration or mechanical shock, internal strain due to regional cooling rate differences, solid particles in the melt at the crystal growth interface, gas bubbles trapped within the melt, surface tension effects or the like. Once generated, dislocations degrade the uniformity of the crystal""s electrical characteristics and permit the attachment of impurities to the single crystal. Further aggravating the problem is that any localized defect or dislocation in the single crystal typically spreads and often renders much of an ingot unusable. Therefore, it is desirable to grow single crystal ingots having the greatest possible zero dislocation length. Ideally, the entire usable portion of an ingot would have zero dislocations.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. In particular, the polysilicon chunks used to charge the crucible have different shapes and sizes because they are typically obtained by manually breaking U-shaped rods of polycrystalline silicon, grown from a chemical vapor deposition process. Due to the brittle nature of polycrystalline silicon and the manual breaking operation, the chunks do not have a fixed shape. Rather, the chunks consist of small and large pieces with a combination of sharp, blunt, and round edges. The shape and size of the chunks, as well as the size distribution of the chunks, can vary widely depending on the producer of the polycrystalline silicon. The sizes of the polysilicon chunks can even vary among lots produced by the same producer.
The size distribution of chunks plays a significant role in the melting behavior of the charge and in the likelihood that dislocations or other defects might appear in the final ingot. For example, the size distribution of polycrystalline silicon chunks influences contributors to loss of structure during the growth process such as splashing rim oxide, flaking, bridging, and quartz pieces. Presently, polycrystalline silicon producers do not measure the size of the chunks or their size distribution on a regular basis. At most, producers of polysilicon use a ruler to measure the length and breadth of the chunks. Unfortunately, this method of measuring chunk size is time consuming, laborious, and inaccurate. Knowledge of size data would facilitate a determination of optimum chunk sizes and size distributions, and, consequently, would improve efficiency and throughput.
For these reasons, an improved system and method for the measuring the average size and the size distribution of polycrystalline silicon chunks for use in the Czochralski process is desired.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved system and method for analyzing the average size and the size distribution of polycrystalline silicon chunks. This is accomplished by an image processor or vision system that is more efficient and less time consuming than the prior art. Advantageously, the present invention provides measurements of a range of parameters including average diameter, perimeter size, surface area, aspect ratio, and the size distribution of the chunks. In addition, the system is economically feasible and commercially practical, and the method can be carried out efficiently and economically.
Briefly described, a method embodying aspects of the invention permits a determination of polycrystalline silicon chunk size for use with a Czochralski silicon growing process. The method includes arranging one or more polycrystalline silicon chunks on a measuring background that provides an image contrast between the polycrystalline silicon chunks and the measuring background. An image of the polycrystalline silicon chunks on the measuring background is generated with a camera. The image has a plurality of pixels and each pixel has a value that represents an optical characteristic of the generated image. The image is processed as a function of the pixel values to detect edges in the image. According to the method, the detected edges are grouped to define one or more objects in the image corresponding to the polycrystalline silicon chunks. The method further includes determining a dimension of each defined object. A size parameter associated with the polycrystalline silicon chunks on the measuring background is then determined as a function of the determined dimensions of the defined objects.
Another embodiment of the invention is a system for determining polycrystalline silicon chunk size for use with a Czochralski silicon growing process. The system includes a measuring background that is positioned to support one or more polycrystalline silicon chunks. The measuring background is such that it provides an image contrast between the polycrystalline silicon chunks and the measuring background. The system also includes a camera for generating an image of the polycrystalline silicon chunks on the measuring background. The image generated by the camera has a plurality of pixels and each pixel has a value representative of an optical characteristic of the generated image. An image processor processes the generated image as a function of the pixel values to detect edges in the image. The image processor groups the detected edges to define one or more objects in the image corresponding to the polycrystalline silicon chunks and then determines a dimension of each defined object. The image processor also determines a size parameter associated with the polycrystalline silicon chunks on the measuring background as a function of the determined dimensions of the defined objects.
Alternatively, the invention may comprise various other systems and methods.
Other objects and features will be in part apparent and in part pointed out hereinafter.